Fluid mixer with rotating baffles



Nov. 28, 1961 H. A. wlsTRlcH EIAL 3,010,803

FLUID MIXER WITH ROTATING BAFFLES Filed June 21, 1956 HARRY A. WISTRICHRICHARD B. OLNEY FIG. 2 BY: MAY/774%@ TH EIR ATTOR NEY United StatesPatent O 3 010,803 FLUID MIXER WI'H ROTATING BAFFLES Harry A. Wistrich,Walnut Creek, and Richard B. Olney,

Oakland, Calif., assignors to Shell Gil Company, a corporation ofDelaware Filed June 21, 1956, Ser. No. 592,946 Claims. (Cl. 23-270.5)

' This invention relates to internally bai-lied, multistage iiuid mixingapparatus suitable, for example, as reactors or as contacting apparatusto effect intimate contact between two or more at least partiallyimmiscible uid phases. When used as a reactor a single iiuid phase maybe passed through the several stages; the device then is a homogeneousreactor. However, several phases may be flowed through the reactorconcurrently. When used as a contacting device, eg., to eiect chemicalreactions or for solvent extraction, two phases are present, and usuallyflow countercurrently through the several stages. All applications areherein generically referred to as uid mixing apparatus.

Mixing apparatus of this type have a series of annular stator battleshaving central openings and mounted at intervals transversely within anelongated vessel, which is usually vertical, and a rotor which includesa shaft extending through the said openings and carrying fast forrotation therewith a plurality of transverse, generally dat rotor balesdistributed among the compartments deiined by the stator baiiies and somounted that they are axially spaced from the adjacent stator baffles,with which they cooperate to form mixing zones. Such apparatus issometimes known as a rotating disc contactor. The general principles ofconstruction and operation and the vortex patterns created by thebaffles when used as a contactor are disclosed in U.S. Patents Nos.2,601,674, 2,729,544 and 2,729,545 and will not be described in detailherein.

As is more completely set forth in the cited patents, the rotor baiesproduce within the several compartments vortex patterns which cause onephase to be dispersed intimately within the other, and the dispersed andcontinuous phases gravitate into adjoining compartments in accordancewith their relative densities. Such gravitating ow is satisfactory whenthe operation involves reasonably balanced net rates of ow of the twophases countercurrently through the vessel. In some operations, however,the net or throughput rate of iiow of one of the phases, either thedispersed or the continuous one, is insufcient to maintain a compositiongradient through the series of compartments when known constructions ofthe apparatus are used. For example, it was found that when one phaseflows only intermittently or very slowly, such as at one-tenth of theflow rate of the other, excessive axial mixing occurs. The compositionof the slowly moving phase then tends to become or becomes uniformthroughout the several compartments, so that the contactor is in effectoperating at a very small number of stages or as a single stage; thisphenomenon occurs although the same contactor can he operated at alarger number of theoretical stages with the identical ratio of phasespresent within the vessel but with increased flow of the above-mentionedphase. The diiculty is especial- 1y severe in uid systems that arediiiicult to mix; in such cases high mixing intensities produced, eg.,by high rotor speeds, are necessary but promote equalization of thecomposition throughout the several compartments.

The situation of low net rate of flow of one phase, considered in thepreceding paragraph, is met with whenever a fluid stream is treated witha disproportionately smaller volume of a treating iluid, eg., whenhydrocarbon oil is treated with sulfuric acid or gasoline is treatedwith caustic.

3,010,803 Patented Nov. 28, 1961 ICC Axial mixing is also a problem whenthe device is used as a reactor lwherein only one phase occurs orwherein several phases flow concurrently, since any passage of uidcounter to the main ow direction results in unequal residence times fordierent parts of the reaction mixture.

It is a general object of the invention to overcome the above-noteddiiculty by arranging the rotor and stator baffles in such a way as tolimit axial mixing. Speciic objects are to limit back-mixing of theslowly moving phase and thereby to increase the number of theoreticalcontacting stages realized when internally baled apparatus of the Vtypeindicated is used as a contacting apparatus and the rate of throughputof one phase is excessively low in relation to the rate of throughput ofthe other phase, especially when a high mixing intensity is required;and to limit back-mixing of the uid passing through the vessel when usedas a homogeneous or concurrentiiow reactor so as to effect a moreuniform residence time.

In summary, according to the invention the general object is attained byusing a non-uniform spacial relation between the stator baies and therotor along the length of the vessel, such that there occur within thevessel some mixing zones having relatively greater intercompartmentfluid intermixing rates than others. The latter zones, herein calledstaging zones, are usually zones of relatively lower mixing intensitiesand limit theginntercompartment fluid intermixing. Various specificarrangements are possible: Por example, stator baille-rotor combinationsor compartments relations providing high intercornpartment intermixingrates may alternate regularly throughout the length of the vessel ywithothers providing low intercompartment intermixing rates; or, a pluralityof consecutive compartments providing similar relations (all havingeither high or low intermixing rates), may be grouped at two or morelevels in the vessel which groups are separated from one another bysingle cornpartments or groups of compartments providing relations ofthe opposite type (i.e., having low or high intermixing rates,respectively).

The variables in the aforesaid relation between the rotor and the statorbales which influence the intermixing rate include the diameter of therotor baie, the diameter of the opening in the stator baille, and theproximity of the rotor bales to the stator hales in the axial direction.In general, for a given angular velocity decreased inter-compartmentfluid intermixing are promoted by the use of smaller-diameter rotorbales (including in the extreme case the omission of the rotor baillewithin a compartment), of stator baflies with smaller openings, and bygreater axial clearances between rotor baflles and stator bailies, therst and last of these expedients being accompanied by decreased mixingintensities. These expedients may be employed singly or in any desiredcombination; the second is preferably employed together with another,such as the iirst. These expedients may in some instances reduce thecapacity of the apparatus as a whole by restricting the forward passageof iluids .between compartments, but nonetheless improve the overalleifectiveness.

.The invention outlined above is founded on the principle that thetendency of a phase, such as the slowlymoving phase in a contactingoperation, to become uniform throughout the series of contactingcompartments is due to back-mixing, in which some of the said phase Howsbetween compartments in the reverse axial direction, i.e., toward theend of the vessel at which it Was admitted. When this axial mixingoccurs at a rate that is high in relation to the net rate of ow orthroughput rate of the phase in question, the composition is more orless equalized among all or several compartments. The

rate of axial or back-mixing can be expressed in terms of theintercompartment tluid intermixing rate, which is defined as the ratioof the reverse iiow of a given uid phase between adjacent compartmentsto the net forward flow of said phase through the vessel. lt is evidentthat the same principle applies in the case of a unifiow reactor; herereverseilow leads to a wider spread between the maximum and minimumresidence times within the reactor.

It should be pointed out that the reduction of intercompartment uidintermixing brought about by any or all of the expediente consideredabove cannot -be applied to advantage to all of the compartments of theapparatus due to the fact that the resulting reduced mixing intensityand, in some cases, the resulting decreased capacity, are then so greatas to have an adverse influence on the overall characteristics of themixer. 'I'he instant invention purports to maintain good mixing orcontacting conditions` in -at least certain portions of the Vessel,herein called the high-intensity mixing zones, and to separate thesezones from one another by others, herein called staging zones, which,although operating under somewhat less favorable mixing conditions,improve the overall operation by reducing back-mixing and therebyinsuring the more uniform ow of all portions of the phase and theexistence of different compositions in the several highintensity mixingzones.

The degree of composition diierence achieved by the -staging zones aswell as the intensity of mixing in the high-intensity zones can beselected in accordance with the requirements, as determined by thenumber of stages and the rate of fluid ow or capacity required and theVnature off-the fluids being mixed. For example, it is possible toemploy'a highly differentiating staging zone (using, for example, anextremely small rotor baille or a consecutive series of small rotorbafes'or even omit ting the rotor in the staging zone) in combinationwith adjoining high-intensity zones which are operated either withV veryintensive mixing so as to operate each of them as a separate physicalstage approaching one theoretical stage in effectiveness, or withhigh-intensity zones wherein mixing is only moderately more intensivethan that used in the staging zones; or staging zones providing onlymoderate composition differences may be provided, again in combinationwith high-intensity zones of either type.

The invention will be described in further detail with reference to theaccompanying drawing forming a part of this specification and showingcertain preferred embodiments by way of example, wherein:

'FIGURE 1 is a vertical sectional view of a contacting apparatusconstructed in accordance with the invention;

FIGURE 2 is a transverse sectional view taken on the line 2 2 of FIGUREl FIGURE 3 is a diagrammatic vertical sectional view showing thearrangement of the stator baiies and the rotor in accordance with theembodiment of FIGURES l and 2; and

FIGURES 4, 5, 6 and 7 are diagrammatic vertical sectional views showingthree alternative arrangements of the stator batlles and the rotor.

Referring to FIGURES l, 2 and 3, the vessel includes a verticalcylindrical wall 10, circular in cross section, having upper and lowerinlet pipes 11 and 12, respectively, which may optionally be tangential,as shown, an upper discharge pipe 13 and, in the bottom wall 14, alo'wer discharge pipe 15. The vessel is closed by a top plate 16carrying an electric motor 17 which is coupled to a central, verticalshaft 18. This'shaft is rotatably mounted in bearings 19 and 20. Aplurality of horizontal, flat, annular stator bales 21 to 21m is mountedimmovably within the vessel at suitable intervals, which are equal inthis instance. iThese baes are imperforate except for central circularopenings 22, which are large "n relation `to the cross section of theshaft. The stator batlles divide the vessel into a vertical series ofcompartments which communicate consecutively through said openings. Theshaft 18 carries `a plurality of rotor baflles 23 to 23] in the form ofat, circular, imperforate discs, and the shaft and rotor bafflescollectively constitute the rotor. Each rotor baille is fixed to theshaft for rotation therewith and is situated within a compartment inaxially spaced relation to the adjoining stator bal-lies, e.g., at themid-level of the compartment. A stator bafe 24 and a current-suppressingelement such as an egg-crate structure 25 may be optionally mountedabove the inlet 11. Further, a tube 26 may surround the upper part ofthe shaft 18. The vessel may have a jacket 27 fitted with pipes 28 and29 for the circulation of a heating or cooling fluid.

Considering new the improvement according'to this embodiment of theinvention, it will be noted that the diameters of the openings in thestator baffles and of the rotor baffles are not uniform; thus, thestator bafles 21 and 21a-d-e-h-i-l-m have large openings and the otherstator bal-lies 21b-c-fgjk have smaller openings. The rotor baffles23b-f-j, which lie between adjoining pairs of stator bal-lies with smallopenings, have smaller diameters than the other rotor bal-lies 23 and23a-c-d-e-g-h-zk-l. The consequence of this arrangement is that therotor baille 23h is related to the adjacent stator baffles 2lb and 21Cin a manner to cause lower mixing intensities and lowerinter-compartment fluid intermixing rates than those caused by stator'baille-rotor combinations on either side thereof, such as between therotor bathe 23 and stator baffle 21a and between rotor bathe 23e andstator baille 21d. Similarly, the rotor bafdes'23f and 23]' are relatedVto their adjacent stator bales to produce lower inter-compartment uidinterrnixing rates.

In the preferred arrangement illustrated the largest rotor batles canpass through the stator battles having the smallest openings therein, sothat the rotor shaft and its baes can be lifted through the series ofstator baties. This arrangement is not however, in every case essentialto the present invention.

The operation as a contacter is yas follows: It is assumed that theheavier fluid is to form a continuous phase and that the lighter fluidis to be dispersed therein. The column is 'filled through inlet 11 withthe heavier uid, the shaft 18 is rotated by the motor 17 in a directionindicated by the arrow A (this direction being preferably the same asthe tangential direction of the inlet pipes 11 and 12) and the lighteruid is admitted continuously or intermittently through the inlet 12, theadmission of heavier fluid at 11 being continued. Toroidal vortices Tare thereby set up within each compartment, resulting in the dispersionof the lighter uid in the heavier. The greater part of the dispersion isrecirculated within the vortices and the balance gravitates from stageto stage. Thus, the net path for a particle of lighter iluid betweencompartments is as indicated by the dashed line L. IThese ow patternsare further described in the above-cited patents. The relatively quieterspace between the lowermost battle 21m contains only the heavier fluid,com-mingled above the inlet 12 with large bubbles or globules fof thelighter fluid. The heavier fluid is discharged through the outlet 15 ata rate controlled by the valve 30. The dispersion rising past the upperinlet 11 has the rotational movement thereof checked by the structure25; this permits the dispersed `droplets to settle upward in a quiescentsettling space above in which it collects above the interface I. Thelighter iluid is withdrawn through the outlet 13 and the heavier Ifluidsettles back through the structure 25, so that there is no net verticalflow thereof at this level.

As was noted earlier, when one of the fluid phases flows slowly orintermittently, back-mixing, i.e., axial mixing thereof in the reversedirection, would tend to equalize among the several compartments thecomposition of that phase if all stator baille-rotor relations were thesame throughout the vessel for rotor speeds sufficient good for mixing.This -tendency is eiectively counteracted by the instant construction inthat the smaller-diameter rotor battles 23b-f-j are so related to theiradjacent higher and lower stator baffles as to cause less intensivemixing; also, the smaller openings in the said adjacent stator bafestend to reduce back-mixing, and this expedient supplements the eiect ofthe smaller rotor bales. 'Ihe entire compartments containing thesesmaller rotors (including the mixing zones immediately above and belowthese rotors) therefore constitute staging zones, which divide thevessel into four high-intensity mixing zones which operate at favorablecontacting conditions and among which materially different compositionsof both the slowly-moving phase and the more rapidly-moving phaseprevail. Thus, the two compartments above the stator baffle 2lb from onesuch high-intensity mixing zone and those between batlies 21C and 21jform another, etc. It may be noted that either the ascending, dispersedphase or the descending, continuous phase may be that which ows throughthe vessel at the lesser rate.

While the foregoing description pertained to the dispersal of thelighter iluid, it is evident that the invention is equally applicable tothe case Where the heavier iiuid is dispersed, as is disclosed in theaforementioned patents. In this case the baille 24 and structure 25 aremounted at the bottom, below the inlet 12, and Ithe interface I issituated below the said structure. Dispersion of the heavier fluid iseiected by admitting it only after the vessel has been iilled with thelighter fluid.

It may be further noted that the invention may be applied to contactorsof various forms, including partie-- ularly those with streamliningbodies or flat guide rings, in accordance with the above U.S. PatentsNos. 2,729,544 and 2,729,545, respectively.

The additional views of the drawing illustrate diagrammatically certainspeciiic modified arrangements whereby the relations between the statorbatles and the rotor can be varied.

In the arrangement according to FIGURE 4, the baffling `arrangementdiiers from that previously described only in that all of the statorbailles 31 are of the same construction. The three rotor bafles 33a-b-chave smaller diameters than the other rotor baflles 33 and thecompartments 37, 38 land 39, wherein the former are situated, formstaging zones; the other compartments form high-intensity mixing zones.Thus, the upper two compartments form one high-intensity mixing zone andthe group of three consecutive compartments beneath that containing therotor 33a form another etc. This arrangement, as well as that of FIGURES1-3, is suitable particularly at relatively low rotor speeds, whereinelective mixing is provided despite a somewhat lower mixing intensity byarranging several high-intensity compartments consecutively, and only amoderate ditferentation of composition between these zones is necessary,e.g., when back-mixing is only moderately severe. The arrangement ofFIGURE 4 permits somewhat greater throughput and would permit slightlygreater back-mixing than that of FIGURES 1 3.

FIGURE 5 shows a construction wherein the rotor baffles are omitted inthe compartments which serve as staging zones; the stator baies 41 areof identical construction throughout -the vessel, but they may bearranged as was described for FIGURES 1 3. All compartments whichcontain rotor baies 43-which may be all of the same diameter-arehigh-intensity mixing compartments, wherein the rotor baflies arerelated to the stator baiiies to eiect eiective mixing. In the othercompartments, indicated by the numbers 47, 48 and 49, the only part ofthe rotor occurring is the shaft, and it is related to the statorbattles to cause very low-intensity mixing, whereby these form stagingzones. Some mixing does, however, occur inasmuch as the dispersion moveswith high rotational velocity, which is for the most part transmittedthrough the fluid from the adjoining compartments which have rotorbaliies and in small part induced -by the shaft.

Since the staging zones provide open ow spaces for the continuous,unobstructed passage of the fluid dispersion therethrough-being freefrom coalescing means or packing, which would obstruct such flow of adispersion-no stratification of the phases occurs therein. These stagingzones achieve a high degree of composition difrerentation and may beregarded as the ultimate in making the rotor bailies smaller. Y

FIGURE 6 shows a contactor arranged to provide three high-intensitymixing compartments 57, 58 and 59',

which contain large-diameter rotors 53a, 53b and 53C,-

respectively; the other compartments contain rotor baffles 53 havingsmaller diameters and hence lower mixing intensities prevail therein, sothat they function as staging Zones. The stator baiiies which liebetween smalldiameter rotor baiiies, Viz., Sla-d-e-h-i-l havesmalldiameter openings, while the other stator bales 51 andSIb-C--g-j-k-m have central openings of larger diameters. Inclusion ofthe terminal stator baffles in the latter group permits .increasedcapacity without flooding. This arrangement is suitable when high rotorspeeds, conducive to high inter-compartment iuid inter-mixing rates, are

used. v

FIGURE 7 shows a variant wherein all stator bafes 61 are alike and twosizes of rotor baiiles are used in alternation. Large-diameter rotorbaies 63 alternate with small-diameter baies 63a. In this arrangementeach compartment which contains a large rotor baie 63 functions as ahigh-intensity mixing zone and the alternate compartments, containingthe baiiies 63a, function as staging zones. Such an arrangement may bemost suitable, for example, for uid systems requiring only moderatelyhigh rotor speeds for good dispersion.

The same considerations apply when the device is used as a reactor.Thus, when used as a homogeneous reactor only one set of ow connections,12 and 13 or 11 and 1S, is needed, and the rotor is used for the purposeof mixing the reacting stream to establish within each compartmentuniformity in composition and temperature, the latter being controlledby the temperature of fluid circulated within the jacket 27.

We claim as our invention:

1. A fluid mixing apparatus comprising: an axially elongated vessel oneend of which is higher than the other; a plurality of transverse,axially spaced, annular stator bai-lies iixed within said vessel, havingopposed faces and central openings, and detining a series ofcompartments which are in consecutive communication through saidopenings; means for admitting fluid to the vessel at one end of theseries and for discharging the fluid at the other end of the series; anda rotor including a rotatably mounted shaft extending through saidopenings with ample radial clearances with respect to the stator baiiiesfor the free passage of duid through the openings and carrying tast forrotation therewith a plurality of rotor baffles distributed among aplurality of said compartments and situated in axially spaced relationto the stator baies, the spatial relations of the stator batlies to therespectively adjacent rotor baiiles being recurrently varying along thelength of the shaft so as to produce a series of mixing zones ofalternating inter-compartment fluid intermixing rates for limitingback-mixing.

2. A iiuid mixing apparatus according -to claim 1 wherein anintermediate rotor baie has a diameter which is less than the diametersof at least some rotor battles situated distributively on opposite sidesof said intermediate rotor baflie.

3. A fluid mixing apparatus according to claim 2 wherein a stator bameadjoining said intermediate rotor bale has an opening the diameter ofwhich is less than the diameters of the openings in at least some statorbattles situated distributively on opposite sides of said intermediatestator bale.

4. A fluid mixing apparatus according to claim 1 wherein an intermediatestator baile has an opening the diameter of which is less than thediameters of openings in at least some stator baffles situateddistributively on opposite sides of said intermediate stator baille.

5. A uid mixing apparatus according -to claim l wherein an intermediaterotor baille has a diameter which is greater than the diameters of atleast some rotor vbailles situated -distributively on opposite sides ofsaid intermediate rotor baille.

6. A uid mixing apparatus according to claim wherein a stator baffleadjoining said intermediate rotor baffle has an opening the diameter ofwhich exceeds the diameters of openings in at least some stator balessituated distributively on opposite sides of said Iintermediate statorbaffle.

V7. A fluid mixing apparatus according to claim 1 wherein anintermediate stator baille has an opening the diameter of which exceedsthe diameters of openings in at least some stator bales situateddistributively on opposite sides of said intermediate stator baffle.

8. A fluid mixing apparatus according to claim 1 'wherein said rotorbaffles yand stator baies are predominantly flat and imperforate savefor said central openings in the stator bai-lies.

9. A iluid contacting apparatus comprising: a vertically elongatedvessel shaped internally as a surface of revolution about a verticalaxis; a plurality of horizontal,

essentially ilat, annular stator bales fixed Within said vessel andextending at vertical intervals f-rom the said interior surface radiallyto central, circular openings, said baffles defining a series ofcompartments which are in consecutive communication through saidopenings; means for admitting uids to be contacted at vertically spacedpoints in said vessel for countercurren-t ow through said compartmentsand for discharging the iluids after such countercurrent flow; a shaftextending `through said openings with ample radial clearances withrespect to the stator bmes for the free passage of fluid through theopenings; means mounting said shaft for rotation about said verticalaxis; a horizontal, substantially flat, circular rotor balie for each ofsaid compartments mounted on said shaft for rotation therewith atlocations spaced by substantial vertical distances from the adjoininghigher and lower stator baes, said rotor batlles including a groupthereof having relatively larger diameters interspersed with rotor balesof relatively smaller diameters.

10. A uid contacting apparatus according to Vclaim 5 wherein thediameters of the central opening in each stator bale is greater than thediameters 'of the rotor baies Ztoward one end of Ithe vessel.

References Cited in the iile of this patent UNITED STATES PATENTS2,601,674 Reman June 24, 1952

1. A FLUID MIXING APPARATUS COMPRISING: AN AXIALLY ELONGATED VESSEL ONEEND OF WHICH IS HIGHER THAN THE OTHER, A PLURALITY OF TRANSVERSE,AXIALLY SPACED, ANNULAR STATOR BAFFLES FIXED WITHIN SAID VESSEL, HAVINGOPPOSED FACES AND CENTRAL OPENINGS, AND DEFINING A SERIES OFCOMPARTMENTS WHICH ARE IN CONSECUTIVE COMMUNICATION THROUGH SAIDOPENINGS, MEANS FOR ADMITTING FLUID TO THE VESSEL AT ONE OF THE SERIESAND FOR DISCHARGING THE FLUID AT THE OTHER END OF THE SERIES, AND AROTOR INCLUDING A ROTATABLY MOUNTED SHAFT EXTENDING THROUGH SAIDOPENINGS WITH AMPLE RADICAL CLEARANCES WITH RESPECT TO THE STATORBAFFLES FOR THE FREE PASSAGE OF FLUID THROUGH THE OPENINGS AND CARRYINGFAST FOR ROTATION THEREWITH A PLURALITY OF ROTOR BAFFLES DISTRIBUTEDAMONG A PLURALITY OF SAID COMPARTMENTS AND SITUATED IN AXIALLY SPACEDRELATION TO THE STATOR BAFFLES, THE SPATIAL RELATIONS OF THE STATORBAFFLES TO THE RESPECTIVELY ADJACENT ROTOR BAFFLES BEING RECURRENTLYVARYING ALONG THE LENGTH OF THE SHAFT SO AS TO PRODUCE A SERIES OFMIXING ZONE OF ALTERNATING INTER-COMPARTMENT FLUID INTERMIXING RATES FORLIMITING BACK-MIXING.